The present invention relates to the field of varactor voltage-controlled oscillator (VCO) circuits and, in particular, pertains to techniques for compensating for the temperature dependence of varactor capacitance.
Varactors, also known as a tuning diodes, have gained popularity for application as voltage-variable capacitors in building VCO circuits. Typically, a VCO circuit includes an LC tank circuit on the input of an oscillator. The capacitive element in the tank circuit may be a varactor. The oscillator frequency is: ##EQU1## where C.sub.vt is the series equivalent capacitance of the varactor.
Varactor capacitance consists primarily of the junction capacitance of a reverse biased PN junction. Accordingly, the capacitance of these devices varies inversely with the applied reverse bias voltage. Therefore, the frequency of the tank circuit, and hence the oscillator frequency, may be adjusted by varying the reverse bias voltage on the varactor.
The varactor has a capacitance of approximately: ##EQU2## where V is the reverse bias voltage across the varactor, phi is the junction contact potential (typically 0.74 volts for silicone at room temperature) and gamma is the capacitance exponent, a function of the doping profile of the varactor device. C.sub.D equals =C.sub.0 where C.sub.0 is the capacitance of the varactor at 0 bias voltage. Phi is a strong function of temperature, for example -2 millivolts per degree C. As a result, the varactor device capacitance drifts substantially over temperature.
Additionally, as indicated in the above equation, the capacitance drift is an inverse function of applied bias voltage. For low bias levels, for example one or two volts, the capacitance drift is as high as +600 parts per million per degree centigrade (PPM/degree C). This represents an oscillator frequency change of -300 PPM per degree C which, at 10 megahertz, means a frequency shift of 3 kilohertz per degree C. Accordingly, a temperature compensation scheme is desirable for any frequency control not using feedback techniques such as a phase locked loop. Temperature compensation is especially important in low bias level applications as the varactor capacitance drift is most pronounced.
The prior art discloses a temperature compensating network for a varactor control signal V.sub.in. The network comprises a forward biased diode in series with the varactor control signal V.sub.in and a bias resistor between the varactor cathode and circuit ground. In operation, an increase in temperature results in a decrease of the forward diode voltage V.sub.diode. If the control signal voltage V.sub.in is constant, the output voltage to the varactor will rise, lowering the capacitance of the varactor, and thereby partially offset the initial capacitance increase caused by the temperature change.
The temperature compensating method described above is impractical in a circuit that operates on a low power supply voltage because of the forward diode voltage drop, typically 0.7 volts at room temperature. For example, in a miniaturized portable FM receiver, the total power supply voltage may be only three volts. Applying the diode drop compensation technique would effectively sacrifice over 23 percent of the potential control voltage swing. Accordingly, the need remains for temperature compensation of a varactor VCO while maintaining voltage swing of the varactor control signal.